Influence of abutment materials on the implant-abutment joint stability in internal conical connection type implant systems

전체 글

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Influence of abutment materials on the

implant-abutment joint stability in internal conical connection type implant systems

Jae-Young Jo1, Dong-Seok Yang1, Jung-Bo Huh1, Jae-Chan Heo2, Mi-Jung Yun1, Chang-Mo Jeong1*

1Department of Prosthodontics, School of Dentisty, Pusan National University, Yangsan, Republic of Korea

2Osstem Dental Implant Research Center (Osstem Implant), Busan, Republic of Korea

PURPOSE. This study evaluated the influence of abutment materials on the stability of the implant-abutment joint in internal conical connection type implant systems. MATERIALS AND METHODS. Internal conical connection type implants, cement-retained abutments, and tungsten carbide-coated abutment screws were used. The abutments were fabricated with commercially pure grade 3 titanium (group T3), commercially pure grade 4 titanium (group T4), or Ti-6Al-4V (group TA) (n=5, each). In order to assess the amount of settlement after abutment fixation, a 30-Ncm tightening torque was applied, then the change in length before and after tightening the abutment screw was measured, and the preload exerted was recorded. The compressive bending strength was measured under the ISO14801 conditions. In order to determine whether there were significant changes in settlement, preload, and compressive bending strength before and after abutment fixation depending on abutment materials, one-way ANOVA and Tukey’s HSD post-hoc test was performed. RESULTS. Group TA exhibited the smallest mean change in the combined length of the implant and abutment before and after fixation, and no difference was observed between groups T3 and T4 (P>.05). Group TA exhibited the highest preload and compressive bending strength values, followed by T4, then T3 (P<.001). CONCLUSION. The abutment material can influence the stability of the interface in internal conical connection type implant systems. The strength of the abutment material was inversely correlated with settlement, and positively correlated with compressive bending strength. Preload was inversely proportional to the frictional coefficient of the abutment material. [J Adv Prosthodont 2014;6:491-7]

KEY WORDS: Preload; Interface stability; Abutment material; Implant; Compressive bending strength

INTRODUCTION

Endosseous dental implant is one of the methods for

restoring partially1,2 or completely3,4 edentulous patients.

The treatment has a high success rate, and is now com- mon.5-7 Most implant systems are composed of an endosse- ous fixture and a prosthesis-supporting abutment, and each component is connected with a screw.8 During mastication, the interface between the prosthesis and implant abutment is subjected to various physiological loads. Should the mag- nitude of such loads exceed the safety limits of the inter- face area, failure due to the loosening of the interface con- figuration or fatigue fractures can occur.9-11 Factors affect- ing the stability of the interface include external loads,12,13 the preload of the abutment screw,14 and the implant-abut- ment connection type.14-18

In order to enhance the stability of the implant-abutment interface, external loads imposed on the screw-retained inter- face should be minimized, and the preload should be maxi- mized.19 In order to reduce the external loads borne by the

Corresponding author:

Chang-Mo Jeong

Department of Prosthodontics, Graduate School of Dentistry, Pusan National University, 20 Geumo-ro, Mulgeum-eup, Yangsan 626-770, Republic of Korea

Tel. 82 55 360 5130: e-mail, cmjeong@pusan.ac.kr

Received 24 February, 2014 / Last Revision 25 July, 2014 / Accepted 5 August, 2014

© 2014 The Korean Academy of Prosthodontics

This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.

org/licenses/by-nc/3.0) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

This study was supported by Clinical Research Grant, Pusan National University Dental Hospital (2014).

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implant-abutment interface, the implant should be located correctly and the prosthesis should be designed to mini- mize the length of lever. Further, occlusion should be designed such that the loads are transferred along the implant axis in order to prevent an excessive concentration of stress at implant-abutment joint.20-22

Abutment screws become elongated owing to torque during tightening, and compressive force is generated between the implant and abutment because of the elastic recovery of the screw. The tensile force thus generated within the screw is called preload.14 As the stability of the implant-abutment interface increases with the increase in magnitude of preload, the maximum possible elongation of the abutment screw within the range of its elastic recovery should be utilized in order to increase the preload.23 Martin et al.24 reported that the higher the tightening torque, the higher the magnitude of preload, and Simon25 have sug- gested that tightening torque should exceed 30 Ncm to reduce loosening of the screw. When a screw is tightened, 90% of the initial tightening torque is directed toward over- coming frictional force and only the remaining 10% is con- verted into preload.26 Frictional force occurs between the screw and fixture, the screw head and abutment, and the abutment end and fixture,24 and if the frictional force between these structures is reduced more force can be con- verted into preload.14 In attempts to reduce the frictional coefficient of the abutment screw, different screw materials have been investigated14,27 as has coating the surface of the screw.14,28 In their study on the influence of screw material and shape on preload, Jőrnéus et al.27 reported that flat- headed gold alloy screws showed the highest preload mag- nitude, and Park et al.28 reported that coating the abutment screw reduced the frictional coefficient, thus increasing the preload.

The implant-abutment interface can be broadly divided into internal conical connection and external hex connec- tion types.29 In the internal conical connection type implant systems, the tightening torque is manifested through the elongation of abutment screws, and also through the taper- lock effect owing to settlement of the conical abutment.

The majority of functional loading is transferred along a tapered interface, resulting in a limited load on the abut- ment screw. Thus, internal conical connection type implants have been reported to have better screw joint stability than the external hex connection type.29-31

While several studies have investigated preload, screw loosening, fracture,14-18 abutment screw materials,14,27 and surface coating14,28 with a view to enhancing the stability of the implant-abutment interface, the influence of abutment materials on the stability of the implant-abutment interface has not yet been sufficiently studied. In this study, we mea- sured and compared settlement, preload, and compressive bending strength before and after abutment fixation in order to investigate the influence of abutment materials on the stability of the interface in internal conical connection type implant systems.

MATERIALS AND METHODS

In this study, internal conical connection type fixtures (GSII, Ø4.0 × H11.0 mm, Osstem Co., Seoul, Korea), cement-retained abutments (GSTA5610, Ø5.0 × H5.5 mm, Osstem Co., Seoul, Korea), and tungsten carbide-coated (CW/C) abutment screws were used (Fig. 1). Five uniform- ly sized abutments for each group were fabricated with commercially pure grade 3 titanium (group T3), commer- cially pure grade 4 titanium (group T4), or Ti-6Al-4V (group TA). A total of 15 sets of fixture, abutment, and

Fig. 1. The GSII implant system used in this study. (A) GS II implant assembly, (B) Fixture, (C) Abutment, (D) Abutment screw.

A B C D

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abutment screw were used in the experiment, and the mate- rials for the implant components are listed in Table 1.

In order to determine the amount of settlement of dif- ferent abutment materials when tightening torque is applied, the length of each abutment screw was measured before and after tightening. Since it is difficult to define the baseline value prior to the application of tightening torque, the abutment was placed on the fixture by applying 5 Ncm tightening torque to the abutment screw with a digital torque gauge (MGT12E, MARK-10 Corp., New York, NY, USA) and the length from the upper top of the abutment to the bottom end of the fixture was measured. Then, an additional tightening torque of 30 Ncm was applied, fol- lowed by re-measuring of the implant length and subse- quent calculation of the difference between the 2 measure- ments (Fig. 2). This process was repeated 5 times for each specimen using a 1-µm precision micrometer (Mitutoyo Corp., Kanagawa, Japan).

Measurement of the preload of an abutment screw must be preceded by measurement of the tensile force of the screw, or the compressive force between the abutment and the implant. In this study, the compressive force between the abutment and the implant was measured by the SlimLine force sensor (9132B21, Kistler Instrument Corp., Novi, MI, USA). If the sensor is placed directly between the abutment and the implant, the difference in screw-tight- ening length and the morphological change in the interface can lead to errors. To prevent this, the implant body model was made using the same material and internal structure to the GSII implant system. The model was made up of the upper and lower parts, and the sensor was placed between them (Fig. 3). The preload was measured while tightening the abutment screw at a tightening torque of 30 Ncm with a digital torque gauge (MGT12E, MARK-10 Corp., New York, NY, USA).

A fixture-implant specimen fastened using a tightening Table 1. Features of the implant-abutment systems

Group Abutment materials Abutment surface Fixture Abutment screw

T3 CP-Ti grade 3

Titanium alloy WC/C coating

T4 CP-Ti grade 4 TiN coating CP-Ti grade 4

TA Titanium alloy

CP-Ti: commercially pure titanium, Titanium alloy: Ti-6Al-4V.

Fig. 2. Measurement of settlement. (A) Before tightening torque application, (B) After tightening torque application.

A B

Fig. 3. Schematic diagram of the preload measuring apparatus. The "a" means implant-abutment interface and

"b" means implant-screw interface. (A) Implant assembly, (B) Implant body model.

A B

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torque of 30 Ncm was fixed to a universal testing machine (Instron8841, Instron Corp., Mass, Norwood, MA, USA) under the test condition of marginal bone resorption of 3 mm as specified in the dental implant test standards (ISO14801). A cylindrical metal tube constituting to the superstructure of the prosthesis was fabricated, with a hemispheric upper end. After cementing the metal tube to the abutment with temporary resin cement (Premier Implant Cement, Premier Dental Product Company, Plymouth Meeting, PA, USA), an inclined load was applied at a loading angle of 30° (ISO14801) at a speed of 1 mm/

min (Fig. 4). The maximum load was measured right before the implant and abutment complex was irreversibly bent.

One-way ANOVA using SPSS (Release 12.0, SPSS Inc., Chicago, IL, USA) was performed in order to assess the

significance of changes in implant length, preload, and compressive bending strength in relation to abutment mate- rials, which was then verified with Tukey’s honestly signifi- cant difference (HSD) post-hoc test.

RESULTS

Table 2 outlines the mean values of settlement, preload, and compressive bending strength measured after abutment fixation. Group TA showed the smallest mean values of settlement, and no differences were observed between groups T3 and T4 (P>.05, Fig. 5). Group TA exhibited the highest preload values, followed by T4 and T3 (Fig. 6), demonstrating significant intergroup differences (P<.001, Fig. 7).

Fig. 4. Schematic diagram of the compressive bending strength test apparatus.

Table 2. Means and standard deviations of the settlement, preload, and compressive bending force

Group (n=5) Settlement (µm) Preload (N) Compressive bending force (N)

T3 53.0 ± 3.9* 316.6 ± 18.8 823.1 ± 9.8

T4 48.8 ± 2.7* 426.0 ± 25.1†† 865.4 ± 7.6††

TA 43.0 ± 1.9** 473.4 ± 23.1††† 912.3 ± 14.1†††

Different numbers of asterisks in the same column indicate significant differences (P<.05, Tukey’s HSD).

Different numbers of daggers in same column indicate significant differences (P<.001, Tukey’s HSD).

T3: Commercially pure titanium grade 3, T4: Commercially pure titanium grade 4, TA: Ti-6Al-4V.

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DISCUSSION

To improve the joint stability of the implant-abutment interface, a higher preload should be produced during screw tightening and preserved when functional load is applied to the prosthesis.32 When tightening torque is applied to the screw or a functional load is applied to the prosthesis, the phenomenon of settlement takes place at the initial contact point, provided that the loading exceeds the yield strength. Once the initial contact surface is flat- tened as a result of morphological changes, affecting the micro surface roughness and shortening the distance between abutment and fixture, the settlement of the abut- ment leads to shortening the abutment screw and loss of preload. 33 Lee and Lee34 reported that the effect of differ- ent abutment materials on abutment settling in the conical connection implant system. In their study, there was no sig- nificant difference between abutment materials. In present study, however, group TA showed significantly less settle- ment (P<.05). Since the abutments used in our experiment are all TiN-coated to improve the esthetic color appeal of the peri-implant mucosa35 and to reinforce the surface hard- ness and abrasion resistance,36,37 the frictional coefficient, surface hardness, and abrasion resistance were expected to be similar among all experimental groups. The abutment materials tested were, in increasing order of strength,38 commercially pure grade 3 titanium (group T3), commer- cially pure grade 4 titanium (group T4), and Ti-6Al-4V (group TA). In the limitations of this study, the amount of abutment settlement under tightening torque is influenced more by the material that the abutments are comprised of than their surface coatings. The phenomenon of settlement occurs not only under tightening torque, but also under functional loading. Therefore, using abutments comprised of high-strength materials may diminish preload losses (i.e., screw loosening) caused by abutment settlement under functional loads and reduce changes in occlusion.33

The preload generates a compressive force between the implant and abutment and resists external loads. External loads imposed on a screw joint cause fine slipping between screw threads, resulting in screw length reduction and thus partial preload loss. If the preload loss progresses to the point that it falls below the threshold value, external loads or vibratory energy trigger the rotation of screw threads, leading to screw loosening.39 Therefore, a higher preload confers proportionally higher resistance against screw loos- ening and fatigue fracture, due to the reduced potential of external load-induced joint separation.14,23 Increasing the preload involves the frictional force as well as the tighten- ing torque of the screw, and there are studies reporting that higher preloads were obtained by reducing the frictional coefficients of abutment screws14,27,28 as in the case of the tungsten carbide-coated screws used in our study. Yet, as well as the frictional force occurring between the abutment and fixture, that occurring between the abutment and screw,24 and the materials that the abutment is comprised of, can be important determinants of the magnitude of Fig. 6. Comparison of mean preload (*P<.001, Tukey’s

HSD).

Fig. 7. Comparison of mean compressive bending force (*P<.001, Tukey’s HSD).

Fig. 5. Comparison of mean displacement (*P<.05, Tukey’s HSD).

Displacement (µm)

70 60 50 40 30 20 10

0 T3 T4 TA Group

Preload (N)

600 500 400 300 200 100

0 T3 T4 TA Group

Compressive bending force (N)

960 920 880 840 800 760

720 T3 T4 TA Group

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preload. Corazza et al.40 reported that the carbon film coat- ing of abutment surface reduced abutment screw loosening.

In their study, the carbon film coating improved hardness, adaptation at the implant-abutment interface and scratch resistance, and reduced the friction coefficient of the abut- ment-screw head interface. In present study, group TA showed the highest preload followed by groups T4 and T3 (P<.001). All three groups were subjected to the same con- ditions, and all of the abutment screws were coated with tungsten carbide; thus, the difference in the materials that the abutments were comprised of is thought to have been responsible for the differences in the frictional force gener- ated between the screw and abutment observed between the groups. As the frictional coefficient of Ti-6Al-4V is smaller than that of commercially pure titanium,41 that the highest preload was exhibited by group TA may be attribut- able to the smaller frictional force generated between the screw and abutment as compared to the other 2 groups, which contributed to an increase in the tightening torque used for elongation of the abutment screw.

The compressive bending strength of the implant-abut- ment interface was highest in the TA group, and lowest in the T3 group, and all groups differed significantly from each other (P<.001). Since the strength of the abutment material used in group TA, which exhibited significantly higher preload values (P<.001), was higher than that of groups T3 and T4, the differences in compressive bending strength evidently resulted from the differences in abut- ment material strength and preload. However, given that in the real-life oral situation loads are not static, rather fatigue is caused by repetitive dynamic loads, further studies are necessary.

CONCLUSION

The results of this study suggest that the abutment material plays an important role in the interface stability of implants of the internal conical connection type. Provided that bio- logical risks can be excluded, it would be recommendable to use abutment materials with high strength and low friction- al coefficients to improve the mechanical stability of the implant-abutment interface.

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